Roll cooling



Dec. 12, 1967 H. E. MULLER 3,357,224

ROLL COOLING Filed Feb. 26, 1965 K 20 50 /LO Lia-r United States Patent O 3,357,224 RLL CGLING Herman E. Muller, Portage, 1nd., assigner to inland Steel Company, East Chicago, 1nd., a corporation of indiana Filed Feb, 2d, 1965, Ser. No. 435,630 Ciaims. (Cl. 22-201) ABSTRACT 0F THE DISCLGSURE Cooling a mill roll by a thin dynamic film of liquid coolant applied against the roll surface under a shoe having a smooth inner surface evenly spaced over a substantial area of the roll surface, the shoe being adapted to move the coolant in a circumferential ilow in intimate heat transferring contact with the roll surface, and adjustable positioning means operating on the shoe to -control the thickness of the dynamic film.

The present invention relates to means and method for cooling moving metal surfaces that are subject to high temperatures over recurrent periods, such as the rolls of a hot rolling mill. Although the invention is particularly addressed to rolling mill rolls for the working of hot metal by way of illustration, the invention is, nevertheless, adaptable to the cooling of any surfaces where comparable conditions obtain, and where a high rate of heat abstraction under controlled conditions that conduce toward thermal equilibrium is desired.

A major problem in a hot strip mill operation is the removal of heat from the mill rolls. These rolls are in high pressure Contact with hot metal slabs, from which they absorb large quantities of heat which must be removed by a cooling fluid such as water. Not only must the rolls be cooled, it is important `for manufacturing that they be maintained at an even temperature.

Various prior cooling structures and methods have been employed, including fluid baths and sprays. However, these prior art methods and apparatus are deficient in several respects, particularly in the eliiciency and controllability of heat removal. They fail to remove heat at a sufliciently high rate and require large volumes of cooling fluid in proportion to the amount of heat removed. These deficiencies are aggravated by the increased demand for closer dimensional controls in modern strip mill operations and by the increased roll heating due to higher operating speeds and longer strips.

It is accordingly a general object of the present invention to provide novel apparatus and methods for fluid roll cooling which overcome the above and other prior art deficiencies.

It is an object of the present invention to provide novel means and methods for rapid and even extraction of heat from a mill roll, which means and methods also provide increased heat removal efficiency.

It is another object of the present invention to provide novel apparatus and methods for fluid cooling a roll in a hot strip mill by which the rate of heat removal may be simply yet accurately controlled, so as to achieve and maintain the optimum roll temperature.

It is a further object of the present invention to provide low cost roll cooling apparatus of high efficiency that may be readily installed on existing hot strip mills.

A more specific object of the invention is to provide new and improved roll cooling apparatus for a hot strip mill wherein a flexible shoe is urged against the working surface of the roll and cooling fluid is injected under pressure between the shoe and the roll surface and is conned therebetween to form a thin dynamic fluid layer flowing in efficient heat transferring contact with the roll surface.

A further specific object of the invention is to provide novel methods of roll -cooling in a hot strip mill wherein cooling fluid is dynamically compressed against a segment of the roll surface to constitute a broad, high velocity, thin film moving circumferentially of the roll, the fluid being controlled to govern the rate of heat removal from the roll in a uniform manner and providing conditions of thermal equilibrium in the roll.

Further objects and advantages of the invention pertain to the particular arrangements and structure whereby the above-identiiied objects and other objects of the invention are attained.

The invention,' both as to its structure and mode of operation, will be better understood by reference to the following disclosure and the drawings forming a part thereof, wherein:

FIGURE l is a cross-sectional side view of an apparatus in accordance with the present invention;

FIGURE 2 is a view of the apparatus of FIGURE 1 along the line 2-2 of FIGURE 1;

FIGURE 3 represents a plot of the velocity of flow of a cooling liuid versus the distances from the surfaces of the shoe and the roll; and

FIGURE 4 is the representation of FIGURE 3, showing the change in velocity prole with a reduction in the distance between the two surfaces.

Turning now to the drawings, and FIGURE 1 in particular, there is shown therein a roll cooling apparatus 10 in accordance with the present invention. The apparatus 10 includes an elongate thin exible shoe 12 adjustably mounted adjacent the cylindrical roll 14 and urged toward continuous Contact with the roll surface by a force applying system 16, including a hydraulically actuated piston and cylinder 44 and a screw follower structure 46. A fluid supply connection manifold structure 20 supplies fluid under pressure to the area between the shoe 12 and the working surface of the roll 14. The fluid is conined as a thin high velocity stream therebetween, the volume of which may be controlled by regulating the force applying system 16 to achieve an optimum clearance between the shoe and roll so that a controllable and highly efcient heat transference between the roll and the cooling fluid is realized.

The shoe 12 is preferably constructed and mounted so as to coextend with and closely overlay a longitudinal segment of the working surface 22 of the roll 14. Thus, the shoe 12 is parallel to and substantially coextensive in length with the longitudinal (axial) length of the working surface 22 of the roll. The width of the shoe 12 preferably covers an acute arcuate segment subtending an acute angle 24 of 20 tov 60 more or less about the roll axis. It is preferred that the shoe 12 be limited in width (arcuate extent) around the circumference of the roll 14 as an excessively wide shoe results in inferior fluid flow characteristics, including a higher pressure drop requiring a higher pressure head. Further, as will be further explained herein, it has been found that it is not sutiicient `for efcient heat removal simply to envelop the roll surface with a blanket of water, as a static water lm is a poor heat transfer medium compared with the dynamic water lm of the present invention.

The shoe 12 preferably has a smooth inner surface 26 in complementary parallel relation to the surface 22 of the roll 14. Further, the shoe has sufficient llexbility accurately to conform the inner surface 26 to the curvature of the roll 14. Where, for example, the shoe 12 is formed from sheet stainless steel or other suitable strong yet flexible material, the inner surface 26 may be one of the originally planar surfaces of the sheet metal. It is preferred that the surface 26 be free Of irregularities and capable of being substantially continuously and equidistantly closely spaced from the working surface 22 of the roll.

A top edge 36 and a bottom edge 38 of the shoe are preferably reinforced to form beam supports. This may be provided, for example, by securing a cylindrical tube to the shoe edge, as shown, or by other suitable means. The purpose in this reinforcing is to provide increased strength and resistance to flexing at the longitudinal axially extending edges 36 and 38. These edges are provided Awith suitable connections for the force applying system 16 to provide the adjustable clearance between the shoe and the roll. The force applied to the edge of the shoe is distributed evenly throughout the shoe.

To allow the cooling fluid to be injected between the shoe and the roll surface, the shoe 12 is preferably provided with fluid apertures 28 therethrough adapted to be connected to and communicate with the fluid supply manifold structure 20. These fluid apertures 2S need not necessarily be spaced cylindrical passageways, as shown, but may be in the form of one or more elongated slots extending across the shoe 12, or other Suitable configurations. It is preferred that the fluid apertures be located centrally between the top edge 36 and the bottom edge 38 of the shoe, and extend linearly along the length of the shoe. This arrangement delivers fluid across the entire working surface of the roll and induces fluid flow circumferential the roll surface.

The fluid supply connection manifold structure as illustrated includes: a nozzle beam 30 extending axially across the outside surface of the shoe and providing a passageway to each fluid aperture 28, a plurality of flexible bellows tubes 32 communicating with each of the fluid apertures 28 and connecting into the nozzle beam 30, and a fixed high pressure water header pipe 34 extending the axial length of the shoe, into which each of the flexible `tubes 32 is connected and supplied from. The nozzle beam 30 provides increased rigidity and linearity to the shoe along the line of the -fluid apertures 28. It is to be appreciated that the above arrangement is merely exemplary and that numerous other fluid supply connection arrangements may be employed.

The fluid -supply source 40, which is connected into and -supplies the water header 34, is preferably a high flow capacity and high pressure source of a cooling fluid such as water, and may comprise any suitable conventional apparatus. Preferably there are conventional means provided for regulating the pressure head, examplified b-y the valves 42.

The force applying system 16 is adapted to force the shoe 12 toward the roll so as to maintain the inner surface 26 of the shoe closely and evenly spaced from the roll working surface 22 when the cooling fluid is forced under the shoe. Various force applying arrangements may be provided other than the preferred arrangement described below.

The preferred force applying system 16 includes hydraulic or pneumatic cylinder-piston actuators 44 acting upon at least one edge of the shoe. Preferably the pistons act on the shoe along a line 43 which is chordal but nonperpendicular to the roll surface. In this manner the shoe is acted upon by force components which force the shoe against the roll surface and also exert tension on the shoe. The exertion of a tension force on the flexible shoe between the top and bottom edges causes the shoe to attempt to achieve a flat configuration. This is resisted by the curvature of the roll surface and hence the inner surface of the shoe is drawn tightly and continuously against the -roll surface.

Preferably there is one cylinder-piston actuator 44 located near each end of the shoe, so as to operate the shoe evenly across the working surface of the roll, Also each cylinder-piston drive 44 is preferably provided with an adjustable mechanical stop 4S thereon limiting the travel of the piston, and thereby setting the minimum clearance of the shoe top edge 36 from the roll surface.

The force applying system 16 shown herein preferably further includes a pair of hand-wheel and screw follower structures 46 or other suitable adjustable fixed supports, each fastened to the shoe near one end of the bottom edge 38. As shown, this hand-wheel and screw arrangement provides precise adjustment of the clearance of the shoe bottom edge 3S from the roll surface. It will be appreciated, however, that the hand-wheels and screws 46 may be replaced by a cylinder-piston drive arrangement similar to that holding the top edge 36. Further, although the force applying system 16 herein shown provides both an adjustable static support and a controlled dynamic support for forcing the shoe against the roll, these functions need not necessarily be combined.

The roll cooling apparatus 1C* may be installed by mounting it to any convenient fixed framework near the roll surface, so that the shoe 12 is positioned over and aligned with the roll working surface. For the structure shown, the hand-wheel and screws 46 holding each end of the shoe lower edge 3S are adjusted to a desired clearance of the shoe from the roll, for example, approximately 1/16 inch. The upper mechanical stops 45 may be adjusted for a similar clearance. Once the minimum clearances are provided they need not be again readjusted.

The actual operation of the apparatus 10 preferably consists generally in controlling the force applied to the shoe by the force applying system and the pressure head of the cooling fluid applied to the shoe, as the rate of heat transfer is primarily a function of the fluid pressure, the space provided under the shoe, and to a lesser extent the fluid inlet temperature (which is generally a constant).

In turning on the apparatus 10 preferably the two cylinder-piston means 44 are actuated first. Aided by the chordal line of action of the pistons with respect to the roll, the flexible shoe is forced toward continuous contact with the working surface of the roll along the line of the fluid apertures 28 and is simultaneously pulled in tension to apply even pressure by the entire shoe surface against the roll. The valves 42 may then be opened so that fluid from the high pressure fluid supply source 40 flows into the water header 34, through the row of flexible bellows tubes 32, into the nozzle beam 3ft, and then through the fluid apertures 28, where the fluid contacts the roll surface and escapes at high velocity between the roll surface and the shoe.

As the applied fluid pressure increases, the apparatus is preferably adapted to allow the fluid pressure under the shoe to force the shoe away from the roll. That is, the water pressure causes the shoe to back away evenly from the roll for a short clearance distance, this distance being dependent upon the water pressure and the dynamic counteracting force applied by the force system 16. However, an increase in the fluid velocity under the shoe lowers the effective pressure exerted by the fluid against the shoe in accordance with the Bernoulli theorem. This assists in maintaining an even and small clearance spacing.

The dynamic thin layer of cooling fluid moves at a high velocity completely confined between the inner surface of the shoe and the roll surface, flowing from the line of apertures toward the top and bottom edges of the shoe where the fluid escapes. In effect, the inside of the shoe and the roll surface form an equidistantiy spaced fluid nozzle between their surfaces. The fluid velocity is substantially constant and high under the entire shoe inner surface, not just near the edges of the shoe where the fluid escapes. The forcible confinement of the cooling fluid to a dynamic film of substantially constant thinness in intimate heat transferring contact with the roll surface provides a highly efficiently and rapid heat transfer from the roll to the cooling fluid.

As the fluid apertures 28 are preferably spaced longitudinally across the roll surface, the fluid discharge path is substantially entirely circumferential the roll,

with only an insubstantial flow discharging axially from the roll at each end (which may be further reduced by packing if desired). This confining of the fluid to a liow circumferentially about the roll is important in providing even and efiicient cooling. In an axial liow the ow paths are longer and therefore the pressure drop is higher. Further, if only a single fluid discharge point were used so that the flow paths would range from purely circumferential to purely axial, the lengths of the fiow paths would vary widely, resulting in very different iiow rates over different areas and therefore uneven roll cooling.

The high velocity circumferential ow provides a further advantage in forcibly wiping or scrubbing the roll surface over the entire substantial segment of the roll surface under the shoe. This not only removes any foreign matter which may be on the roll but also strips the roll surface of any possible residual and insulative film of cooling fluid. it is important to remove such residual iiuid, as a static film prevents efficient cooling and must be removed to permit uniform heat exchange with the dynamic fluid so as to maintain thermal equilibrium.

By increasing the hydraulic or pneumatic pressure applied to the cylinder-piston actuators 44 the force applied against the shoe can be increased to force the shoe inner surface 26 closer to the roll surface. This reduces the thickness of the iiuid film, increases the dynamic properties, and thereby improves the fluid eliiciency by the increased quantity of heat transferred per unit of applied cooling fiuid. However, for high rate heat removal there must be a relatively high rate of fiuid fiow and accordingly the thickness of the fluid layer cannot be so greatly reduced so as to restrict the flow below the optimum rate needed to maintain a dynamic thermal equilibrium. increasing the thickness of the fluid layer (by decreasing the force against the shoe) and/or increasing the applied iiuid pressure head will increase the fluid iiow rate and therefore increase the cooling o f the roll. It may thus be seen that by relating the fluid inlet pressure with the force applied against the shoe by the force applying system, the roll cooling apparatus may ne rapidly and easily controlled to vary the rate of heat removal and hence control the temperature of the roll. Dynamic thermal equilibrium may be achieved and maintained. The velocity of iiow may be controlled in the same manner so that the fiow is either above or below laminar fiow conditions, as may be desired.

The rate of heat removal and the heat removal eiiiciency can be easily determined fromthe difference in the fluid temperatures at the inlet and outlet (discharge) together with the weight or volume of iiuid used per unit time. These values may be conventionally measured.

Considering the novel cooling efciency of the roll cooling apparatus 10, reference is made particularly to FIG- URES 3 and 4, which represent idealized plots 48 and 49 of the velocity of flow profile of the cooling fluid flowing between the roll surface and the inner surface of the shoe. A velocity flow profile is ymore or less parabolic between closely spaced extended parallel surfaces, and it may be seen that the distance between the two surfaces 22 and 26 directly affects this velocity profile.

The apparatus of the invention differs from previou-s water cooling arrangements. By regulating and controlling the water film thickness and velocity, extremely high rates of heat transfer under controlled conditions can be achieved.

It is not enough to envelop the roll surface with a blanket of water. A static Water film provides relatively little and inadequate cooling for a strip mill roll. Strip mill rolls accumulate heat by cyclic strip contact. In order to attenuate roll thermal conditions or keep the roll in thermal equilibrium the water fiow must be rapid and uniform adjacent the heated surface and cover the pass area.

The present invention permits regulation of the ow of a water iilm so that it approaches iiow -conditions obtained between smooth planar plates positioned very close together. The more closely spaced the plates, the higher the values of critical iiow velocity. It is desirable to keep the flow laminar or non-turbulent because under these conditions more eihcient liow talres place. The orderly orientation of water filaments makes for better heat transfer because there is freedom from eddy currents, turbulence and back circulation.

Contrasting the two situations in FIGURE 3 and FIGURE 4, there is a significant change between the velocity profiles 38 and 39 with a reduction in the spacing of the two surfaces. For a condition of equal maximum flow 50, it may be seen that at the same distance 52 from the roll surface the velocity 53 in FIGURE 4 i-s significantly greater than the velocity 54 in FIGURE 3. This illustrates that control of the water film thickness is essential to optimum heat abstraction. Under conditions of laminar flow only the molecules in the stream passing close to the plate, having intimate molecular contact with the plate, are heated directly. More remote molecules of water in the stream are heated only indirectly and no not pick up as much heat. It has been observed in prior devices that while large volumes of water have been applied to the rolls, very little heat abstraction results, as evidenced by the fact that the departing stream of water is not substantially warmer than the implinging stream. Thus it is important both for maximum heat removal and for heat removal efficiency to achieve a maximum possible (critical) iiow velocity as close as possible to the roll surface, rapidly removing the water which is actually being heated. The heat removal efciency is increased because an increased percentage of the total cooling water is heated.

In describing the invention it is not implied that flow must always be laminar. Under certain conditions, where it is necessary to have a very high heat abstraction rate, the Water head or pressure at the manifold may be raised so as to result in higher than critical (turbulent) flow.

The apparatus and methods described herein provide highly effective, eflicient and controllable lroll cooling, and are presently considered to be preferred. However, it is understood that numerous variations and modifications may be made therein by those skilled in the art and it is intended to cover in the appended claims all such variations and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for water-cooling a roll in a hot strip mill comprising: a liexible shoe mounted adjacent the working surface of said roll, said shoe having a smooth and continuous inner surface which i-s coextensive in area with and overlies a substantial acute arcuate segment of said roll surface and is substantially coextensive in length with the working axial length of said roll, said shoe being adapted to allow said inner surface to be evenly spaced from said roll surface, and said shoe having reinforced longitudinal axial edges; force means connecting with said shoe and adapted to controllably hold said inner surface of said shoe evenly spaced from said roll surface, said force means including piston means acting against at least one of -said reinforced edges along a line non-perpendicularly chordal to said roll surface; cooling water supply means providing for continuously injecting cooling water between said inner surface and said roll surface under pressure including water aperture means in said shoe and movable water connection means `communicating with said aperture means, said water aperture means extending centrally substantially the entire length of said inner surface; said shoe and said force means being adapted to forcibly confine said cooling water to flow circumferentially of said roll in a thin film of even thickness and even velocity between said inner surface and said roll surface in continuous intimate heat transferring contact with said roll surface and to control said thickness of said film to maintain a condition of optimum efficiency heat removal and dynamic thermal equilibrium.

2. The apparatus of claim 1 wherein said shoe and said force means are adapted to control said thickness of said film to achieve a maximum laminar ow velocity.

3. Apparatus for cooling a mill roll comprising: a shoe mounted adjacent the surface of said roll, said shoe having a smooth inner surface which corresponds in area to a substantial minor area of said roll surface and is adapted to be evenly spaced from said roll surface, said shoe having fluid aperture means therein extending along the avial length of said shoe adapted to provide for injecting cooling fluid between said inner surface and said roll surface under pressure; adjustable positioning means adapted to maintain said shoe closely adjustably spaced from said roll surface; said shoe being adapted to forcibly confine said cooling fluid to ow continuously circumferentially of said roll between said inner surface and said roll surface in an evenly thin layer in intimate heat transferring contact with said roll surface whereby heat may be evenly and efficiently removed from said roll by said cooling fluid.

4. The apparatus of claim 3 wherein said adjustable positioning means includes independently adjustable means engaging the opposing circumferential edges of said shoe, and wherein at least one of said independently adjustable means is a dynamic force means urging said shoe toward said roll surface.

5. The apparatus of claim 4 wherein said shoe is flexible.

6. Apparatus for cooling a mill roll comprising: a shoe mounted adjacent the working surface of said roll, said shoe having a smooth and continuous inner surface which is coextensive in area with and overlies a substantial acute arcuate segment of said roll surface and is substantially coextensive in length with the working axial length of said roll, said shoe having uid aperture means therein adapted to provide for continuously injecting cooling fluid between said inner surface and said roll surface under pressure, said fluid aperture means extending along said shoe for a distance substantially coextensive with the length of said inner surface; positioning means supporting said shoe and adapted to controllably position said inner surface of said shoe evenly from said roll surface; said shoe being adapted to forcibly coniine said cooling fluid to flow continuously circumfcrentially of said roll between said inner surface and said roll surface in a controllably thin even layer in continuous intimate heat transferring contact with said roll surface, whereby heat may be evenly, efficiently and controllably removed from said roll by said cooling fluid.

7. A method for efficiently cooling a roll in a hot strip mill with a cooling iiuid comprising the steps of: applying said cooling fluid under pressure onto the working surface of said roll; forcibly confining over an arcuate segment of said roll surface all of said fluid in a thin dynamic llayer of uniform thickness and uniform velocity flowing circumferentially of said roll in intimate heat transferring contact with said roll surface, and regulating the rate of heat removal from said roll by controlling the rate of flow of said fluid in said thin dynamic layer and the thickness of said thin dynamic layer.

8. A method for roll water-cooling in a hot strip mill comprising the steps of: covering only an arcuate segment of said roll with a uniformly thin dynamic film of water; controlling said film to a uniformly circumferentialy oriented flow over said roll surface; and controlling sad uniform film thickness by variable dinamic compression so as to control the velocity of said film about a uniform critical velocity for optimum cooling elhciency under conditions of thermal equilibrium.

9. The method of cooling moving metal surfaces that are exposed to recurrent periods of high heat input, such as the rolls of a hot rolling mill, which includes exposing a preselected area of the surface to be cooled to liquid coolant introduced thereto in a controllab'e ow, confining all of said flowing liquid coolant in a dynamic uniformly thin firn throughout said preselected area, moving the surface to be cooled past said preselected area in continuous contact with said dynamic thin film, maintaining the ow of said coolant by discharging it predominately from opposite sides of said preselected area to orient the main flow thereof in directies coincident with the direction of movement of the surface to be cooled, and controlling the flow of said coolant and the thickness of said dynamic thin film so as to abstract heat from said surface at rates conducive to thermal equilibrium.

10. A device for rapidly abstracting heat from moving surfaces comprising a first means for confining a uniformly thin dynamic film of a liquid coolant against a surface to be cooled, means for injecting liquid coolant to said first means to form said film, said rst means being constructed and arranged to conduct and discharge said film along and in intimate heat transferring contact with said surface predominately in direction of the surfaces movement, and means associated with said first means for controlling the thicknesses of said film toward a critical fiow velocity in said film to maintain optimum efficiency heat removal.

References Cited UNITED STATES PATENTS 2,033,046 3/l936 Montgomery 72-201 3,031,872 5/1962 Kusters 72-200 1,978,895 10/1934 Clark 72-201 FOREIGN PATENTS 914,725 7/ 1954 Germany.

CHARLES W. LANHAM, Primary Examiner.

H. D. HOINKES, Assistant Examiner. 

1. APPARATUS FOR WATER-COOLING A ROLL IN A HOT STRIP MILL COMPRISING: A FLEXIBLE SHOE MOUNTED ADJACENT THE WORKING SURFACE OF SAID ROLL, SAID SHOE HAVING A SMOOTH AND CONTINUOUS INNER SURFACE WHICH IS COEXTENSIVE IN AREA WITH AND OVERLIES A SUBSTANTIAL ACUTE ARCUATE SEGMENT OF SAID ROLL SURFACE AND IS SUBSTANTIALLY COEXTENSIVE IN LENGTH WITH THE WORKING AXIAL LENGTH OF SAID ROLL, SAID SHOE BEING ADAPTED TO ALLOW SAID INNER SURFACE TO BE EVENLY SPACED FROM SAID ROLL SURFACE, AND SAID SHOE HAVING REINFORCED LONGITUDINAL AXIAL EDGES; FORCE MEANS CONNECTING WITH SAID SHOE AND ADPATED TO CONTROLLABLY HOLD SAID INNER SURFACE OF SAID SHOE EVENLY SPACED FROM SAID ROLL SURFACE, SAID FORCE MEANS INCLUDING PISTON MEANS ACTING AGAINST AT LEAST ONE OF SAID REINFORCED EDGES ALONG A LINE NON-PERPENDICULARLY CHORDAL TO SAID ROLL SURFACE; COOLING WATER SUPPLY MEANS PROVIDING FOR CONTINUOUSLY INJECTING COOLING WATER BETWEEN SAID INNER SURFACE AND SAID ROLL SURFACE UNDER PRESSURE INCLUDING WATER APERTURE MEANS IN SAID SHOE AND MOVABLE WATER CONNECTION MEANS COMMUNICATING WITH SAID APERTURE MEANS, SAID WATER APERTURE MEANS EXTENDING CENTRALLY SUBSTANTIALLY THE ENTIRE LENGTH OF SAID INNER SURFACE; SAID SHOE AND SAID FORCE MEANS BEING ADAPTED TO FORCIBLY CONFINE SAID COOLING WATER TO FLOW CIRCUMFERENTIALLY OF SAID ROLL IN A THIN FILM OF EVEN THICKNESS AND EVEN VELOCITY BETWEEN SAID INNER SURFACE AND SAID ROLL SURFACE IN CONTINUOUS INTIMATE HEAT TRANSFERRING CONTACT WITH SAID ROLL SURFACE AND TO CONTROL SAID THICKNESS OF SAID FILM TO MAINTAIN A CONDITION OF OPTIMUM EFFICIENCY HEAT REMOVAL AND DYNAMIC THERMAL EQUILIBRIUM. 